I have a client that wants to attach many (20?) sound sensors to a Mega and relay the data to Processing for interesting effects.I'd like to get a simple 2-wire sensor that doesn't require building a circuit.

Please: what would you recommend? I know nothing about audio processing, be gentle.

You can't digitise 20 channels of audio and pass it along the serial port at the same time. You will be doing well to manage one at indifferent quality.Processing is not good for audio processing, it is not designed for it, it is designed for graphics. But it depends on exactly what is needed.

I want to capture the average volume at each sensor a few times a second. Even a low sampling rate will probably be enough.The idea is that people will be moving about near the sensors, which will in turn change the visual effect on a screen some distance away.